1. Field of the Invention
The present invention relates to an optical beam scanning device to scan a light flux from a light source onto a photosensitive surface of a photosensitive body in a main scanning direction, and particularly to a technique to realize an improvement in optical characteristics of a scanning light.
2. Description of the Related Art
Hitherto, there is known a technique in which in an optical beam scanning device which shapes a light flux a from light source into a specified sectional shape and scans it in a specified direction, a lens having a negative power or a diffractive optical element to allow light fluxes from plural light sources to pass through is provided in a pre-deflection optical system, so that when a temperature change occurs, a beam interval is changed in a direction of suppressing a color shift occurring in an image forming apparatus including the optical beam scanning device (JP-A-2005-221870).
Besides, there is also known an optical beam scanning device in which a resin lens having one surface of an anamorphic refractive surface and the other surface of an elliptical power diffractive surface is arranged in front of a reflecting surface of a deflector, and the power of the power diffractive surface is set so that a variation of position (beam waist position) where a beam diameter in a main scanning direction and/or a sub-scanning direction is converged becomes substantially zero (JP-A-2006-154701).
Besides, there is also known a structure in which a diffractive lens having a diffractive surface to correct a chromatic aberration of magnification is provided in a post-deflection optical system, so that the length of a scanning line in a main scanning direction does not fluctuate according to wavelength variation (JP-A-2006-171117).
Besides, there is known a structure in which an optical element in a post-deflection optical system is constructed of refractive surfaces and diffractive surfaces, the radius of curvature of at least one refractive surface thereof in a sub-scanning direction is continuously changed correspondingly to a main scanning direction from a position on an axis to a position outside the axis, and the diffraction power of at least one diffractive surface in the sub-scanning direction is continuously changed correspondingly to the main scanning direction from a position on the axis to a position outside the axis, so that the F number (Fno), in the sub-scanning direction, of the light flux incident on the surface to be scanned is made substantially constant in an image effective area (JP-A-2002-221681).
However, in the structure disclosed in JP-A-2005-221870, there is a case in which the object can not be achieved according to the combination of materials of a housing of the optical beam scanning device, a frame to position the plural photosensitive bodies, and a shaft to drive an intermediate transfer belt for superimposing developed images on the plural photosensitive bodies and the temperature distribution. Besides, according to a condition, the defocus change amount at the time of temperature change can not be sufficiently suppressed.
Besides, in the structure disclosed in JP-A-2006-154701, since the light beam passes along the optical axis of the resin lens in the sub-scanning direction, the optical path in the sub-scanning direction can not be changed according to the temperature change, and the color shift caused by the thermal expansion of an image forming apparatus can not be corrected.
Besides, in the structures disclosed in JP-A-2006-171117 and JP-A-2002-221681, since it is not considered to correct the position shift of a scanning line in the sub-scanning direction caused by the thermal expansion of an image forming apparatus, the color shift caused by the temperature rise of the image forming apparatus occurs.